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ok a while back i wrote a post about my check engine light coming on on my 2002 325ci .i ran the code and it was my coolant level, thermostat or a sensor.i filled my coolant and the light went off.every once in a while it will come back on and go off maybe take 2 weeks to go off again but then will stay on for a day and then shut off.recently my yellow radiator dummy light pops on and pops off.now i add coolant to where the floating pin should be at the filler neck.then the light will come on again maybe a day or two later and the floating pin in the filler neck will be at the bottom.where is my coolant going?i dont see any leaks.im a little confused was thinking about doing a complete coolant flush and having them put in the bmw coolant just to be on the safe side.any advice or help would be greatly appreciated guys thanx

Likely your problem is the temp sensor O-ring in the lower radiator hose. Very common problem, even if it is not leak, it needs to be replaced. No tools needed, very little coolant lost as system vacuum keeps coolant from leaking.

Technically the O-ring is not a serviceable part.

See my signature for the O-ring part number.

In a pinch, you can pull sensor, remove O-ring, 4-5 wraps of teflon tape in O-ring groove, reinstall.

If the switch is wet at all or if there is any wetness on the lower engine cover it is leaking.

You can clean up the residue you see, fill up to proper level, remove underskirt, and find out where your leak is. Jfoj might be right that it's the 0-ring...which, if leaking, is worth the 'expensive' IAT o-ring...but it could be leaking elsewhere. Clean things up, run car, and find source of leak.

Or, as you said, just replace everything. If you don't want to replace everything, you need to diagnose what's leaking...simple as that.

Check the entire system...I like using my 100 lumen flashlight for looking at stuff...you won't believe what a difference having proper light makes to clear diagnosis.

Likely your problem is the temp sensor O-ring in the lower radiator hose. Very common problem, even if it is not leak, it needs to be replaced. No tools needed, very little coolant lost as system vacuum keeps coolant from leaking.

Technically the O-ring is not a serviceable part.

See my signature for the O-ring part number.

In a pinch, you can pull sensor, remove O-ring, 4-5 wraps of teflon tape in O-ring groove, reinstall.

If the switch is wet at all or if there is any wetness on the lower engine cover it is leaking.

Other than that, expansion tank is a likely suspect.

Great thread guys....I know my o-ring is leaking, I can see fluid around it.
I have a new o-ring already but when the sensor is removed does fluid pour out?
Dont want to drain my rad just yet. I'm pretty sure my expansion tank is also leaking to up by
the top of the rad. Light is coming on and can see fluid around exp tank connections area.

@Peterock29 , i seem to have the same problem as well . No trace of a leak , no heating up and no engine oil & coolant mix-up but am adding a lot of coolant like every 2 days and i feel that the car has lost power especially when i accelerate . Any progress on your part ?.

Crazy One , what's the progress with your car ?. Mine the mechanic had to bleed the coolant system again as he suspected that at one point it might have been opened whilst hot thereby leading to air being clogged in . since then the level hasnt dropped but am not too comfortable with the performance & also the sound of the engine when on idle , there is a funny sound with some bit of shaking as well .

Crazy One , what's the progress with your car ?. Mine the mechanic had to bleed the coolant system again as he suspected that at one point it might have been opened whilst hot thereby leading to air being clogged in . since then the level hasnt dropped but am not too comfortable with the performance & also the sound of the engine when on idle , there is a funny sound with some bit of shaking as well .

I changed it out and no more leak from the temp sensor. It was an easy fix for a cheap price.
Unfortunately I also saw coolant on the radiator near the coolant fill spout.
I found what looks like a crack in the side plastic piece on the radiator tank.
I ordered a new expansion tank before noticing the crack so when I change the radiator I will also install the new expansion tank.
Might as well do it all at the same time.

Yeah I read here that there is a specific procedure to bleeding the air out properly.
I have not noticed a change in the engine but it may be that I just haven't noticed any.